The spa market, especially for coextrusion is growing rapidly. Sheet extruders are seeking ways to create new designs, colors and effects, in an effort to differentiate their products in this growing market.
Current spa units are manufactured by thermoforming a cast acrylic sheet, extruded monolithic polymethyl methacrylate (PMMA) sheet, coextruded ABS/PMMA, or ABS/ASA sheet. The cast acrylic sheet used in these applications can exist in many different textures and colors, including those having a uniform distribution of color and mineral particles, such as described in U.S. Pat. No. 5,705,552; and U.S. Pat. No. 6,562,927; and a variegated non-homogeneous granite appearance as disclosed in U.S. Pat. No. 7,250,472.
There are several drawbacks to using a cast sheet in spa manufacture, as compared to using extruded sheet. In a cast-sheet process, it is far more difficult to change colors and textures than in an extrusion in which a larger variety of colors and appearances can be manufactured. The extrusion process allows for direct production of a multi-layer sheet, which can include the particle layer, as well as substrate and cap layers in a single multi-layer extrusion process. Extruded sheet also can be made to produce a more textured surface during thermoforming than an article made form a cast sheet. This extra flexibility in manufacturing provides a greater ability to change or control the surface texture by changing the cap thickness, or by embossing the surface with a patterned roll. The production of sheet by an extrusion process, also allows for an even distribution of polymer particles and metallic compounds, due to the increase and efficient mixing, without a time to settle during further polymerization of a matrix syrup—as in a cast process.
U.S. Pat. No. 6,841,612, incorporated herein by reference, describes a simulated mineral article composed of crosslinked polymer particles in a thermoplastic matrix, the composition being capable of multiple passes through extrusion or molding. It was found that the levels of the crosslinked polymer particles in this patent had to be lowered when used in combination with the metallic and/or opalescent materials, to produce useful sheet.
Surprisingly it was found that extruded sheet having an evenly distributed combination of crosslinked particles, and metal or opalescent materials in a thermoplastic matrix can be produced having the desired appearance and textures. The extruded layer is protected by a cap layer. The particle-containing appearance layer could be extruded by itself to form sheets and profiles, or could be co-extruded or multi-extruded, for instance over a substrate and capped by the transparent cap layer. The extruded sheet can then be thermoformed into a final article.